1. Field of the Invention
The present invention relates to a sensor for measuring a flow velocity or a flow rate of fluid as a result of detecting change in resistance values of a plurality of heating elements disposed at an upstream side and a downstream side of flow of the fluid.
2. Description of the Related Art
As such a kind of sensor, a flow-velocity sensor or a flow-rate sensor has been known. Such a sensor is provided with a substrate having a depression therein and a film forming a bridge portion or a diaphragm portion which is disposed on an opening plane of the depression. The substrate is placed in flow of fluid. On the film, a pair of heating elements, through each of which an electric current flows, are disposed at an upstream side and at a downstream side of the flow of the fluid, respectively. In the case of a flow-velocity sensor, the resistances of the heating elements disposed at the upstream side and at the downstream side, respectively, are converted into voltages, respectively. As a result of obtaining the difference between changes in the thus-obtained voltages per unit time period, the flow velocity of the fluid is measured.
Such a sensor is disclosed in, for example, Japanese Laid-Open Patent Application No. 5-157758 `Temperature-Characteristics Correction Method for Flow-Velocity Sensor` and Japanese Laid-Open Patent Application No. 9-89619 `Thermal Flow Meter.`
An arrangement disclosed in Japanese Laid-Open Patent Application No. 5-157758 has, as shown in FIG. 1, a depression 52 formed at the center of a semiconductor substrate 51. A diaphragm portion 52a made of a film is formed on the substrate 51 so as to cover the opening plane of the depression 52. On the diaphragm portion 52aa heating element 53 and a pair of temperature-sensing resistance elements 54, 55, disposed at both sides of the heating element 53, are formed. The heating element 53 is heated as a result of an electric current flowing therethrough. Fluid flows in the direction of the arrow F shown in the figure. The temperature of the temperature-sensing resistance element 54 disposed at the upstream side decreases as a result of the temperature sensing resistance element 54 being cooled by the flowing fluid. The temperature of the temperature-sensing resistance element 55 disposed at the downstream side becomes higher than that of the upstream-side temperature-sensing resistance element 54 as a result of the temperature-sensing resistance element 55 being in contact with the fluid which is heated by the heating element 53. Then, the resistances values of the temperature-sensing resistance elements 54, 55, which resistance values correspond to the temperatures of the temperature-sensing resistance elements 54, 55, respectively, are converted into voltages. Then, as a result of obtaining the difference between changes in the thus-obtained voltages per unit time period, the flow velocity of the fluid is detected.
In an arrangement disclosed in Japanese Laid-Open Patent Application No. 9-89619, as shown in FIG. 2, a depression 57 is formed in a surface of a semiconductor substrate 56, bridge portions 58, 59 made of films are formed so as to cross the opening plane of the depression, and heating elements 60, 61 are formed on the bridge portions 58, 59, respectively. Then, the heating elements 60, 61 are heated as a result of electric currents flowing therethrough. Fluid flows in the direction of the arrow F shown in the figure. The temperature of the heating element 60 disposed at the upstream side decreases as a result of the heating element 60 being cooled by the fluid. The temperature of the heating element 61 disposed at the downstream side becomes higher than that of the upstream-side heating element 60 as a result of the downstream-side heating element 61 being in contact with the fluid heated by the upstream-side heating element 60. Then, the resistances values of the heating elements 60, 61, which resistance values correspond to the temperatures of the heating elements 60, 61, respectively, are converted into voltages. Then, as a result of obtaining the difference between changes in the thus-obtained voltages per unit time period, the flow rate of the fluid is detected.
The temperature-sensing resistance elements 54, 55 shown in FIG. 1 are formed so that the lengths thereof along the direction perpendicular to the direction in which the fluid flows are uniform, and the line width thereof is uniform. Also, the heating elements 60, 61 shown in FIG. 2 are formed so that the pattern density of each heating element is uniform along the direction perpendicular to the direction in which the fluid flows. The resistance values of the temperature-sensing resistance elements 54, 55 and the heating elements 60, 61 represent the temperatures thereof, respectively. Each of these temperature-sensing resistance elements 54, 55 and heating elements 60, 61 will be referred to as a heating element, hereinafter. In each of these heating elements, only a small amount of heat is carried away by the substrate at the central portion in the longitudinal direction perpendicular to the direction in which the fluid flows because the central portion is far away from the substrate. A large amount of heat is carried away by the substrate at each end portion in the above-mentioned longitudinal direction because each end portion is close to the substrate. As a result, as shown in
FIG. 6B, where the horizontal axis represents the position on the heating element and the vertical axis represents the temperature of the heating element, the temperature distribution is such that the temperature is high at the central portion of the heating element and the temperature becomes lower at the position closer to each end. As a result, the average temperature of the whole heating element is low. As a result, especially, increasing of the temperature of the heating element disposed at the downstream side of the flow of the fluid is not performed efficiently. When a large electric current is caused to flow through the heating element for the purpose of improving the sensitivity of the sensor, the temperature at the central portion of the heating element is locally high. As a result, the life of the heating element becomes shorter, and, thereby, the life of the sensor becomes shorter.